Mountain Lake has suffered the ravages of civilization since the late 1700s. By now it's a toxic stew of zinc, petroleum hydrocarbons from motor oil and pesticides runoff.

Mountain Lake has suffered the ravages of civilization since the late 1700s. By now it's a toxic stew of zinc, petroleum hydrocarbons from motor oil and pesticides runoff.

Photo: Liz Hafalia, The Chronicle

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Ecological restoration intern for the Presidio Trust Jonathan Young pulling in nets at Mountain Lake at the Presidio in San Francisco, Calif., on Monday, November 5, 2012. The Presidio Trust is gill netting fish at Mountain Lake to remove non-native carp and large-mouth bass to help restore the lake. less

Ecological restoration intern for the Presidio Trust Jonathan Young pulling in nets at Mountain Lake at the Presidio in San Francisco, Calif., on Monday, November 5, 2012. The Presidio Trust is gill netting ... more

Photo: Liz Hafalia / The Chronicle

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Ecological restoration intern for the Presidio Trust Jonathan Young (left) and intern Dave DuBose (right) pulling up nets in Mountain Lake at the Presidio in San Francisco, Calif., on Monday, November 5, 2012. The Presidio Trust is gill netting fish at Mountain Lake to rid non-native carp and large-mouth bass to help restore the lake. less

Ecological restoration intern for the Presidio Trust Jonathan Young (left) and intern Dave DuBose (right) pulling up nets in Mountain Lake at the Presidio in San Francisco, Calif., on Monday, November 5, 2012. ... more

Photo: Liz Hafalia, The Chronicle

Image 7 of 7

Intern Dave DuBose (left) and ecological restoration intern for the Presidio Trust Jonathan Young (right) rowing out to pull up nets in Mountain Lake at the Presidio in San Francisco, Calif., on Monday, November 5, 2012. The Presidio Trust is gill netting fish at Mountain Lake to rid non-native carp and large-mouth bass to help restore the lake. less

Intern Dave DuBose (left) and ecological restoration intern for the Presidio Trust Jonathan Young (right) rowing out to pull up nets in Mountain Lake at the Presidio in San Francisco, Calif., on Monday, ... more

Photo: Liz Hafalia, The Chronicle

Mountain Lake invaders fished out

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Gill nets and a tippy rowboat are being used this month to pluck thousands of alien critters out of San Francisco's Mountain Lake as part of an innovative effort to resurrect the area's ecological history.

The Presidio Trust is trying to remove all of the invasive fish, turtles and other non-native species - some of them dumped from home aquariums - before dredging begins at Mountain Lake early next month.

The idea is to restore the historic body of water as close as possible to the way it was when American Indians roamed its banks. The plan by the trust, the National Park Service and the Golden Gate National Parks Conservancy is to remove toxic sludge, improve water quality and reintroduce native species to the 4-acre pond on the southern boundary of the Presidio.

"This is a huge opportunity for scientific research and education in an urban environment," said Jonathan Young, a San Francisco State University graduate student and biological researcher, after he rowed ashore the other day holding nets festooned with at least 500 squirming juvenile carp and large-mouth bass.

Young, who is spearheading the fish removal project for the trust, said the project is an experiment in urban landscape engineering the likes of which the city has never before seen.

Mountain Lake, which scientists say is 1,700 years old, is one of the few remaining natural lakes in San Francisco and was once a valuable source of drinking water for the Ohlone Indians and European settlers. It is now little more than a sludge-filled pool, festering with leeches, midge fly larvae and other unsavory creatures that like murky water.

Dredging sediment

The heavy lifting is scheduled to begin in early December, when 15,600 cubic yards of sediment will be dredged from the bottom of the lake. The project, which is expected to cost $9.5 million, including $5.5 million in mitigation from Caltrans, will increase the depth of the lake from 8 feet to 11 feet and remove mud that is saturated with lead that seeped in from adjacent Park Presidio Boulevard.

But the lake deepening and toxic sludge removal project is only part of the overall scheme, which was conceived in 2000. The Presidio Trust, with help from thousands of volunteers, has been planting native grasses and cutting groves of litter-prone eucalyptus trees in the area. They are planning to restore historic wetlands, plant native trees and place bio-swales - natural catch basins - next to the lake to redirect the silty runoff from surrounding neighborhoods and allow it to percolate into the ground. When the dredging is done early next summer, workers intend to reintroduce native aquatic plants, fish, turtles and mussels, but Young said all the existing species that replaced them must first be removed.

"The idea is that a healthy lake will have higher densities and greater abundance of invertebrates, and that will bring more birds and bats, but everything is riding on the removal of these fish," Young said.

Origins of problem

The trouble at Mountain Lake actually began in 1776 when the Spanish established el Presidio de San Francisco. The water at that time was crystal clear and clean enough to drink. It was home to western pond turtles, a fish called the three-spined stickleback, the red-legged frog, the chorus frog and a native mussel called the California floater, a filter feeder that improved water clarity, said Young, who has studied historical records, including sediment core samples going back 2,000 years.

The Spanish were the first to cut the native trees, burn wood next to the lake, plant nonnative tree species and graze cattle and sheep, all factors that altered lake habitat. The footprint of humanity only got bigger from there.

Turtles, frogs in soup

Many of the native frogs and turtles were cleaned out during the Gold Rush so restaurants could serve their tasty legs or put them in soup. Legend has it that turtle soup stopped being served in San Francisco during Prohibition because the recipe called for brandy.

In 1853, a 3,500-foot-long tunnel was built next to the lake in a much-touted but failed attempt to bring water to the developing city of San Francisco. A long wooden flume later brought water from the lake around Black Point (now Fort Mason) into the city.

Then, in 1897, the U.S. Army began siphoning water out of the lake to, among other things, irrigate its newly built golf course.

Originally 30 feet deep, the lake shrank by 40 percent when Park Presidio Boulevard was built through part of the lake bed in 1939. Lead from car emissions seeped into the water for decades until leaded gasoline was phased out beginning in the 1970s. The lead combined with zinc, petroleum hydrocarbons from motor oil and pesticides flowing in from the golf course to form a toxic stew.

Invasive species have repeatedly been added to the mix, first by the Army and then by locals setting their pets free. Crawdads, which eat the native aquatic vegetation, and large-mouth bass, which gobble virtually any animal they can swallow, were introduced. Gambusia, a mosquito-eating fish that also eats the eggs of native amphibians, were also added. In recent decades, goldfish, koi (otherwise known as carp), bullfrogs and pet turtles were plopped into the water, where they began to breed. In 1996, an alligator was found in the lake, creating a sensation until it was removed.

The natives have long since died out, poisoned by chemicals, eaten or pushed aside by introduced predators. The last record of a western pond turtle was in 1966.

The restoration plan began in earnest in July when more than 10,000 fish were removed. That included two or three dozen adult carp and bass ranging in size from 20 to 30 inches long, some weighing as much as 15 pounds. In addition, 42 non-native turtles were taken out, all of them pet store species like red-eared sliders, map turtles and painted turtles, Young said.

Relocating reptiles

All of the captured animals are taken to Sonoma County Reptile Rescue, which partners with breeders, pet stores, herpetological societies and local citizens to relocate more than 1,000 reptiles, amphibians and fish a year, ranging from large pythons to tiny newts.

"All the bass go to one pond and the carp to another pond," said Alan Wolf, the Reptile Rescue director, adding that 95 percent of the animals are placed in contained waterways on private property. "We have various people in the county with their own ponds where they keep different fish or turtles. The only thing we require is that any pond with a large number of turtles be fenced off so they can't get out."

Nevertheless, a handful of turtles and thousands of fish remain in Mountain Lake. The problem, Young said, is that a single female carp can produce 2 million eggs in a single season, making it difficult to capture them as fast as they reproduce.

"It's a challenge, definitely," Young said.

Young said other methods might have to be used, such as the introduction of koi herpes or genetically modified carp. Poison would be a last resort, he said, but that option will remain on the table for any stragglers after the dredging is completed.

Meanwhile, biologists are studying possible introductions of species like the San Francisco forktailed damselfly, the western toad and native San Francisco newts.

There is reason for optimism. Habitat restorations elsewhere in the Presidio have led to marked increases in the amount of wildlife, including thousands of baby western chorus frogs earlier this year in a restored seasonal pond near Mountain Lake. The native tree frogs had disappeared from the Presidio many years ago and now appear to be making a comeback.

"This natural area has huge potential to inspire the next generation to really try to protect and conserve and become aware of the biodiversity and our connection to it," Young said. "Urban ecology has been an up-and-coming new field, and with this project, the public is starting to become aware of the impacts of urbanization and the plight of these species."

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